1. Field of the Invention
The present invention relates to an adaptive array antenna.
2. Description of the Related Art
In recent years, a broad band high speed radio communications have been put into intensive practical use. As one example, there is provided a subscriber radio access system. A patent application of an adaptive array antenna for controlling the phase shift of an IF local signal with a phase shifter in this system has been filed by the present inventor (U.S. patent application Ser. No. 09/310198). When the adaptive array antenna is used in a base station of the radio system, directivity can be scanned to obtain the position of each terminal. Moreover, the directivity is changed in a direction of each terminal every time transmission/reception is performed with each terminal. Therefore, interference waves coming from directions other than the terminal direction having a low directivity gain can be suppressed.
FIG. 1 shows one example of such an adaptive array antenna. The adaptive array antenna comprises: a control circuit 1; a transmission IF signal generation circuit 2 connected to the control circuit 1; a divider 3 connected to the transmission IF signal generation circuit 2; a plurality of phase control circuits 4 connected to the divider 3; filters 5 connected to the respective phase control circuits 4; buffer circuits 6 connected to the filters 5; a local oscillator 7 for converting a transmission IF signal to an RF signal for transmission; a divider 8 connected to the local oscillator 7; a plurality of frequency converters 9 connected to respective divided output terminals of the divider 8 and buffer circuits 6; filters 10 connected to the frequency converters 9; buffer circuits 11; filters 12; antenna array elements 13; and a phase shift amount control circuit 14 connected to the phase control circuits 4.
Transmission information formed in an information block is formed as consecutive data packets, and the transmission IF(intermediate frequency) signal generated by the transmission IF signal generation circuit 2 based on the data packet supplied from the control circuit 1 is distributed to the plurality of phase control circuits 4 via the divider 3.
On the other hand, a phase shift coefficient for a transmission destination user of the data packet is sent to the phase shift amount control circuit 14 from the control circuit 1, and the phase shift amount of the phase control circuits 4 is changed for each user.
The transmission IF signal output from the phase control circuits 4 is sent to the frequency converters 9 via the filters 5 and buffer circuits 6. The frequency converters 9 use a local signal sent from the local oscillator 7 via the divider 8 to convert the transmission IF signal to an RF(radio frequency) signal. The RF signal is transmitted from the antenna array elements 13 via the filters 12.
The phase shift amount is determined for each user in this manner, and a radio wave is transmitted via the antenna array elements 13, so that the radio wave can be transmitted in a direction toward the user with satisfactory directivity.
On the other hand, for the subscriber radio access system (base station), a time division multiple access (TDMA) system is generally used in reception, and a time division multiple (TDM) system is used in transmission.
In the TDMA reception system, the radio waves transmitted from a plurality of user terminals are received, and the data packet is reconstructed for each user. However, since the respective user terminals exist in different distances from the base station in most cases, a time difference exists in a radio wave reaching time. Therefore, a region called a guard time for absorbing the time difference is taken between the respective data packets.
On the other hand, since each user terminal receives the radio wave (downlink signal) transmitted from the base station in the TDM transmission system, it is unnecessary to consider the difference of the time for which the radio wave reaches each terminal. Generally from a viewpoint of transmission efficiency, no guard time is positioned.
FIG. 2 shows a generalized format of a downlink frame of a radio communication system in which the TDM system is used.
A control packet is positioned in a top of the frame, and followed by data packet 1, data packet 2, . . . data packet N for separate users. The control packet comprises a header, control information (SI, and the like), and an error correction code (FEC). Each data packet comprises a header, data, and FEC. The control packet includes assignment of a communication channel, request for frequency change, order for communication stop, and the like. Additionally, the control packet of an uplink frame includes a request for user registration, request for communication, request for communication stop, and the like. Examples of a header content include a transmitter radio station ID, destination radio station ID, synchronous capturing signal, and the like.
In this manner, the TDM frame does not include the guard time usually included in the aforementioned TDMA frame. Therefore, when the TDM frame is transmitted via the conventional adaptive array antenna as shown in FIG. 1, a phase shift of the phase control circuits 4 for each user must be performed by a speed sufficiently smaller (faster) than an inverse number of a baud rate because of absence of the guard time.
Here, quadrature modulator ICs are frequently used as the phase control circuits 4. In the quadrature modulator IC, the phase shift amount changeover speed depends on a bandwidth of an Ich/Qch BB signal input. The bandwidth is about 20 MHZ. However, the baud rate is as much as about 21 Mbps in the subscriber radio access system, and the speed cannot be set to be sufficiently smaller than the inverse number of such a high speed baud rate in the phase control circuits 4 formed of the quadrature modulator IC. In the TDM system having no guard time, when the destination user of the packet changes, several bits (header) in the top of the packet are still changing in the phase shift amount in a worst case when the packet passes through the phase control circuits. When the transmission IF signal generated based on the packet passes through the phase control circuits 4 in this state, the transmission direction determined by the phase control circuits 4 cannot be estimated because the phase shift of the phase control circuits 4 is not completed, and the signal is not transmitted to a desired destination in some case. This causes an interference wave in the whole system, and as a result, it is possible that frequency utilization efficiency is greatly influenced.
As described above, for the conventional adaptive array antenna, since there is no guard time in the frame format of the radio communication system using the TDM transmission system, the phase control circuit having a sufficiently high operation speed is necessary. However, to raise the phase shift amount control speed, the control speed needs to be set to be sufficiently smaller than the inverse number of the baud rate. In this case, there is a problem that an IF local signal phase shift circuit satisfying such conditions is expensive.
Accordingly, the present invention is directed to method and apparatus that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
In accordance with the purpose of the invention, as embodied and broadly described, the invention is directed to an adaptive array antenna comprising:
array antenna elements;
first and second phase control circuits which control phase shift amount of a transmission data packet and supply the transmission data packet to the array antenna elements;
a distributor configured to distribute the transmission data packet to one of the first and second phase control circuits based on a destination user information of the data packet; and
a phase shift amount control circuit configured to control the phase shift amount of the first and second phase control circuits based on the destination user information of the data packet distributed to the first and second phase control circuits.
In accordance with the purpose of the invention, as embodied and broadly described, the invention is directed to a transmission method of an adaptive array antenna comprising:
receiving a data packet and destination user information of the data packet;
determining whether or not the destination user information of the data packet is identical to the destination user information of a preceding data packet;
distributing the data packet and the destination user information to a path which is different from a path to which the preceding data packet and the destination user information are distributed;
setting a phase control amount to a phase control circuit based on the destination user information;
activating the phase control circuit;
generating a transmission burst intermediate frequency signal based on the distributed data packet and supplying the generated transmission burst intermediate frequency signal to the phase control circuit; and
converting the transmission burst intermediate frequency signal output from the phase control circuit to a radio signal to be transmitted from the adaptive array antenna.
According to an aspect of the present invention or embodiments consistent with the present invention, a phase shift amount can be securely changed for each user even without using a high-speed and expensive phase control circuit.